This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Basin modeling is the process of using either proprietary or commercially available software to assess charge risk by integrating diverse geological and engineering data types into a model of one or more petroleum systems active in an area being explored. The scale of the model may range in size from a single drilling prospect to an entire basin.
There are several commercial basin simulators that are widely used in academia and industry. These known simulators base the kinetics of petroleum generation on some form of a series of parallel first order reactions, usually expressed by the Arrhenius Equation as a discrete spectrum of activation energies (Ea). These models comprise a distribution of Ea's at fixed or variable spacing (typically 1 kcal/mole) using a single or varying frequency factor (A). The parameters for these models are based on various laboratory experiments. Some known models rely on simple compositional models that are rooted in a petroleum modeling computer program called PMOD developed and made available by the Lawrence Livermore National Laboratory. Additionally, some models offer compositional yield models based on forms of pyrolysis, including open-system pyrolysis. Other models use a compositional yields model based on open and closed-system pyrolysis. Models that predict compositional yields as defined by varying chemical lumps are expressed typically by the bulk kerogen yield kinetic model (discrete Ea's fixed or varying A) where the percentage of each chemical lump is defined for each individual Ea.
One known basin simulator has models of petroleum generation/expulsion that are either two component (gas/oil) or three component (dry gas/wet gas/oil). Generation is modeled using standard discrete energy kinetics derived from open-system pyrolysis for main-stage oil/gas generation and a late-gas generation model for methane formation from reacted kerogen residue. Both processes are modeled using the first order kinetics with a distributed Ea and fixed A. The expulsion model is based on filling available rock pore space to a defined threshold. Secondary cracking reactions are Oil→Gas in the two-component model or Oil→Wet Gas and Wet Gas→Dry Gas in the three-component model. Hence, only the gas/oil ratio or GOR changes as a function of oil cracking with the oil remaining at a fixed composition and quality. In reality, the composition of petroleum changes in a systematic fashion with increasing thermal stress. Polar compounds and large polynuclear aromatic hydrocarbons or PAH are depleted, while the distribution of the surviving hydrocarbons shifts toward smaller species. The net-effect on oil quality is an increase in American Petroleum Institute gravity (referred to as API gravity herein) and GOR and a decrease in sulfur and nitrogen.
Many known modeling techniques employ data that is derived from laboratory experiments. A potential problem relating to laboratory data involving high temperatures is that the coupled processes of generation, expulsion, and secondary cracking may be perturbed and follow mechanisms and pathways that do not occur under geologic conditions. For example, open system pyrolysis experiments measure gas-phase products that are transported from the source rock place in a heated sample holder to a flame ionization or mass spectrometer detector. Hence, expulsion is controlled mostly by volatility. In closed-system pyrolysis experiments, product yields may be defined in terms of gas-phase volatility, solubility in organic solvents, or as free-floating bitumen in hydrous systems. However, the factors that control petroleum expulsion and chemical fractionation in geologic settings, such as kerogen retention and relative solubilities are typically greatly perturbed by the high temperatures required for generation under observable time intervals (typically less than three days though experiments lasting up to five years have been conducted).
Since expulsion and generation cannot be reliably simulated as a coupled process, laboratory experiments have great difficulty in resolving primary generation from secondary, potentially non-realistic, reactions. In closed system experiments, it is often very difficult to resolve whether the measured products evolve directly from kerogen decomposition or from the thermal cracking of confined products that are not normally retained in a natural system environment. The artificial confinement of metastable species also may promote condensation reactions, forming new organic solids. Open system experiments pose the opposite effect; that is, the rapid removal and quenching or detection of metastable products that may remain in natural systems and undergo secondary cracking reactions. Laboratory simulations typically fail to account accurately the retention that occurs in natural geologic systems at much lower temperatures and removal of products as kerogen matures under geologic conditions. Research has shown that all laboratory artifacts cannot be eliminated and that some models may produce unrealistic bitumen compositions.
If laboratory experiments are inherently flawed, an accurate prediction of the thermal decomposition of kerogen and its product yields rests in constructing a theoretical framework based on fundamental principles. That is, the definition of mechanisms and kinetic rates of elementary reactions, which may be measured with high confidence, to molecular assemblages that reflect the complexity of kerogen. With such a model, it should be possible to predict the generation and expulsion under various laboratory and geologic conditions. While there have been numerous studies that formulated reaction schemes and associated kinetics to explain the results of specific laboratory experiments or field observations, there have been few attempts at constructing a comprehensive model that can account for petroleum generation and expulsion under a wide range of conditions. Some attempts have been made at constructing reaction models that account for product yields from experiments conducted under a variety of laboratory conditions and apply the model to a geologic setting. However, these models are phenomenological and rely on the extrapolation of laboratory-derived kinetics to geologic heating rates. An improved method of modeling basin performance, including predicting petroleum production, is desirable.